In North America and many other countries, each broadcast station will be loaned one 6 MHz channel to implement Digital Television Terrestrial Broadcasting (DTTB). Broadcasters have full control of that channel and have the freedom to provide any kind of services. Traditionally, terrestrial television is a fixed service, which requires stationary reception. DTTB systems are able to provide robust audio, video, and data services to fixed as well as mobile recipients using different spectral efficiencies. A DTTB system is able to provide reliable high speed data or video services to roof mounted directional antennae at a spectral efficiency of 3 to 4 bits/s/Hz and at 1 to 2 bits/s/Hz to mobile recipients.
DTTB over VHF/UHF using single carrier modulation suffers from severe multipath distortion due to signal reflections from surrounding objects—passive multipath distortion—and signals transmitted from other transmitters—active multipath distortion. In single carrier modulation (SCM) schemes a single carrier modulates an entire bit stream of digital data and the single carrier is spread over the entire bandwidth of the transmission channel. Furthermore, a signal modulated by a single carrier modulation scheme is susceptible to frequency selective fading. If an in-band spectrum notch occurs, the transmission link could be lost.
In order to overcome the drawbacks of the single carrier modulation Orthogonal Frequency Division Multiplexing (OFDM) has been implemented for digital broadcasting. For example, U.S. Pat. No. 5,757,760 issued May 26, 1998 to Sugita discloses a communication system, wherein the energy of each bit of the inputted information bit string is diffused over the whole frequency band of the orthogonal carriers. In a further advanced system disclosed in U.S. Pat. No. 6,175,550 issued Jan. 16, 2001 to van Nee the bit rate of the transmitted information can be changed by scaling the OFDM process. However, both systems are not able to transmit multiple bit streams with different spectral efficiencies.
Recently OFDM has been implemented in two DTTB standards, the European DVB-T standard and the Japanese ISDB-T standard. The OFDM is a form of a multi-carrier modulation (MCM) scheme, where data are transmitted in parallel in frequency domain on a plurality of sub-carriers. This allows recovery of the data even if some carriers are distorted. Therefore, the transmission of digital data using MCM is less susceptible to multipath distortion. In OFDM the sub-carriers are orthogonally allocated such that they do not interfere with each other. To provide some flexibility, it is possible to include in the DVB-T system hierarchical coding allowing transmission of two separate data streams. A “High priority” data stream is transmitted using modulation of high spectral efficiency and a “Low Priority” data stream is transmitted using low spectral efficiency. One of the weak points of this approach is that the reception of the “Low Priority” data requires a high Carrier to Noise (C/N) ratio resulting in reduced coverage. Therefore, hierarchical modulation has never been implemented in Europe. The ISDB-T system uses band segmented OFDM segmenting the RF channel into 13 segments. Up to three different modulations are implemented on different segments for different services. However, this approach counteracts the principle of the OFDM modulation to spread a data signal modulated onto parallel sub-carriers, preferably, over an entire RF channel to achieve frequency diversity, which gains robustness against multipath distortion. If there is severe fading over a part of the spectrum of the RF channel one or more of the 13 segments are notched out without any chance to recover the data.
It is, therefore, an object of the invention to overcome the drawbacks of the above systems in providing multiple bit streams with different spectral efficiency.
It is further an object of the invention to provide a method and system for interleaving in frequency domain OFDM sub-carriers over the entire spectrum of a RF channel, wherein sub-carriers of different bit streams have different spectral efficiency.